includes an assessment of evaporation rate and is recommended for certain drug products with volatile ingredients (including water). For example, it may be informative to evaluate the water activity for a topical product that is an emulsion containing less than 50% weight/weight (w/w) of water, because differences in manufacturing processes may impact the interactions between the multiple phases in the formulation. Similarly, it may be informative to measure the drying rate for an alcohol-based gel that is expected to evaporate rapidly following topical application. However, neither water activity nor drying rate may be relevant for a petrolatum-based ointment.